Electric motors whose dimensions are often in the range of only a few centimeters are used as actuating drives in many technical fields, for example robotics, automotive engineering, or materials handling. In such applications, it is usually necessary to acquire information regarding the spatial position of the element driven by the electric motor. This positional information can relate, for example, to the opening width of a gripper or to the height of an electrically adjustable vehicle seat. Based on this positional information, a control system can determine, for example, how many revolutions the electric motor must perform, in order to move the element in question into a new desired position.
Numerous concepts have been developed for determining the position of such elements driven by actuating motors. On the one hand, it is possible, of course, to sense the spatial position of the driven element directly. Taking the example of the motor vehicle seat whose height is electrically adjustable, this would mean that its height is measured directly, with respect to a fixed reference point, using a suitable sensor apparatus. This type of direct position sensing is very complex in many cases, however, since the sensor apparatus must be specifically adapted to the particular intended use.
Concepts in which the spatial position of the arrangement is determined only indirectly, via the angular position of the electric motor shaft, are therefore more favorable. This type of indirect position determination is possible whenever the element is driven in slippage-free fashion by the electric motor. Depending on the application, it may be sufficient, in this context, merely to ascertain the number of shaft revolutions that have taken place since a reference point in time. If the requirements in terms of position determination accuracy are more stringent, the angular position of the shaft within a single revolution can also be sensed, and used to determine the position of the driven element.
Sensing, in this fashion, both the number of shaft revolutions and the shaft's angular position within one revolution means simply that the absolute angular position is determined. For example, two revolutions plus 22° in the positive rotation direction corresponds to an absolute angle of 742°. Once the correlation between the spatial position of the element driven by the electric motor and the absolute angle has been determined, e.g. by calibration, a knowledge of the absolute angle therefore allows the position of the element to be inferred.
Electronic revolution counters can be employed in order to sense the absolute angle of the shaft of an electric motor. They exploit the fact that precisely those electric motors used as actuating drives are often electronically commutated. In electronic commutation, the angular position of the shaft is sensed accurately (albeit over only one revolution) so that depending on the instantaneous relative spatial arrangement between stator and rotor, the motor can thereby have voltage optimally applied to it. The absolute rotary encoders used for this furnish a periodic output signal, so that, by means of a suitable electronic analysis system, the number of revolutions can also be ascertained. An early encoder is disclosed in U.S. Pat. No. 3,772,675, now expired.
A disadvantage of this approach, however, is the fact that, following a supply voltage interruption, although the rotary encoder is capable of once again sensing the absolute angular position of the shaft, the information as to how many revolutions the electric motor shaft has already completed is lost, as a result of the supply voltage interruption. As a consequence, the actuating drive must then be recalibrated, using reference point data or the like; this entails high cost. It is possible, in principle, to resort to a voltage supply with battery backup, but this results in additional manufacturing costs and moreover requires regular maintenance, since the batteries must be replaced or recharged on a scheduled basis.
In order to eliminate these disadvantages associated with electronic revolution counters, it has now become common to use rotary encoders that, in mechanical fashion, not only sense the angular position within one revolution of the shaft, as is the case with so-called single-turn rotary encoders, but also allow a determination of the absolute value of the angular position over a number of revolutions. Such rotary encoders are often referred to as “multi-turn” encoders, expressing the fact that the angular position can also be determined absolutely over several turns or revolutions. These mechanical multi-turn rotary encoders reduce the motor rotation speed in one or more transmission stages. The output of each transmission stage is sensed using suitable sensors, and as a result the absolute angular position can be accurately and reliably ascertained independently of the voltage supply. The functional principle is thus similar to that of a mechanical analog clock, in which it is likewise possible, based on the angular position of the second, minute, and hour hands, to ascertain the absolute angle of the second hand over the last 24 hours.
A multi-turn encoder of this kind is known from DE 198 20 014 A1, which was the priority document for PCT/EP99/03056, whose US national phase matured into corresponding U.S. Pat. No. 6,542,088, BIELSKI et al, issued Apr. 1, 2003. This mechanical rotary encoder substantially comprises a single-turn rotary encoder that serves to ascertain the angular position within one revolution, and a multi-turn rotary encoder unit for determining the number of revolutions. In this known apparatus, the single-turn rotary encoder is constituted by a code disk which is directly coupled to the shaft whose angular position is to be determined. The code disk bears a coding which can be scanned in opto-electrical, magnetic, capacitive, or inductive fashion, and with which one revolution of the shaft is divided into a plurality of differentiable sectors. This coding is scanned by a scanning device which generates a multi-digit code word that is correlated with the absolute angular position. The multi-turn rotary encoder unit of the known rotary encoder comprises a reduction gear linkage, coupled to the shaft, whose output is sensed in angular terms by a single-turn rotary encoder. That output is connected to a second reduction gear linkage whose output is likewise sensed using a single-turn rotary encoder. The aforementioned elements of this apparatus are arranged one behind another in the axial direction, the input and output of each reduction gear linkage being, in particular, oriented coaxially; this corresponds to the usual design of small reduction gear linkages of this kind.
Multi-turn rotary encoders of this kind based on reduction gear linkages have the disadvantage, however, as compared to electronic revolution counters, that their dimensions are quite large (at least by comparison with electric motors). In many applications in which electric-motor actuating drives are used, however, the available installation space is very small, so that these mechanical multi-turn rotary encoders cannot always readily be integrated. Another multi-turn rotary encoder is described in DE 10060574-A1, which was the priority document for PCT/EP01/10966, filed Sep. 22, 2001, whose U.S. national phase is Ser. No. 10/181,536, published as US 2003/0112157 on Jun. 19, 2003.